1. Field of the Invention
This invention relates to electrical generators, and more particularly to generators having bearings for supporting the rotor shaft and to a system for detecting the imminent failure of such bearings and the need for bearing replacement.
2. Description of the Prior Art
In electrical generators, such as those used to power the electrical systems of aircraft, the generator is connected to the main engine by a drive shaft. The drive shaft is connected to the rotor assembly through the rotor shaft The rotor assembly is mounted on the rotor shaft and rotates within the stator assembly. The rotor shaft is supported for rotation in the generator housing using sealed bearing assemblies comprising grease lubricated ball bearings. In the course of normal generator operation, the bearings are subject to wear. Eventually, the bearings can wear sufficiently that bearing failure can occur, causing the generator to stop functioning. If the generator stops functioning, the aircraft electrical system may not operate.
In order to prevent bearing failure and to maintain proper operation of the generator, the bearings were usually replaced at predetermined service intervals. The bearing replacement intervals were determined based upon worst-case scenarios of bearing wear, and the bearings were scheduled for replacement before any reasonable possibility of bearing failure could occur. As a result, the bearings were usually replaced long before they were actually worn out, and generators were disassembled and serviced unnecessarily before there is any immediate need for bearing replacement. This resulted in unnecessary servicing, cost and equipment downtime. In addition, it is still possible for bearings to fail before schedule service, and if this occurs, the electrical system failure on the aircraft can be serious.
These problems have been overcome in part by the bearing failure detector described in U.S. Pat. No. 5,602,437, which discloses a conductive ring having an internal opening approximately the same size as the auxiliary bearing clearance. The ring is connected through a lead wire to a control unit. When the main bearings fail, the shaft contacts this ring, and, since the shaft is grounded, the control unit senses that the ring is grounded, indicating bearing failure.
While the bearing failure detector of U.S. Pat. No. 5,602,437 provides a much needed indication of bearing failure, there are still several problems with this detector. The signal produced by the detector is temporary and intermittent, since it depends upon the contact of the shaft with the detector, and the shaft wears away the detector""s conductive ring. When the ring is sufficiently worn, the generator shaft, now supported by the auxiliary bearings, no longer contacts the detector, and the failed bearing signal ceases.
Furthermore, the bearing failure detector of U.S. Pat. No. 5,602,437 does not provide any indication that the detector is actually working. The lack of a ground signal could mean that the main bearings have not failed, but it could also mean that the detector itself has failed. The absence of a ground signal, therefore, does not provide positive evidence of proper bearing operation.
The present invention provides an improved onboard means for determining whether bearing failure is about to occur in aircraft generators so that proper bearing replacement can be undertaken when necessary. Using the present invention, a signal is provided when the main bearings are operating properly an d when the bearing failure detector is operating properly. Thus, the present invention provides a positive indication that the bearing failure detectors are operating properly prior to bearing failure.
The bearing failure detector of the present invention also provides a continuous signal indicating bearing failure rather than a temporary or intermittent signal. This allows for a more reliable detection of bearing failure than was possible with intermittent signals of the prior art.
The bearing failure detector of the present invention provides an improved bearing failure signal which eliminates the need for automatic scheduled replacement of bearings according to a maintenance schedule that may not reflect the need for actual bearing replacement or may not catch a bearing failure before it occurs. The bearing failure detector allows the bearings to operate until they are close to failure, at which time the detector provides a positive indication that the bearings are about to fail and must be replaced within a certain number of hours of operation. This indication may be in the form of a signal to the cockpit to inform the pilot of the need for bearing replacement.
After the bearing replacement indication is provided, the present invention allows the shaft to ride upon the auxiliary bearings until the bearing assemblies are replaced. The auxiliary bearings have a slight gap between their inside diameter and the rotor shaft and are only used when the main bearings have worn to the point that their internal clearances have sufficiently enlarged to equal this gap.
The bearing failure detector of the present invention includes a disc located within the bearing liner next to the auxiliary bearings and near the location of the main bearings. A thin metal ring is plated or formed as a film on the inside diameter of the disc. The ring has a space or gap, so that it is discontinuous around the inner circumference of the disc, and a pair of leads are connected to the ring, one lead on each side of the space or gap. A closed circuit is thus formed through the two leads and around the ring, and is present before the bearing has failed and while the detector is operating properly. This closed circuit is monitored by a control circuit at all times and is used as a bearing function signal, indicating that the bearings are operating properly and supporting the shaft. The ring is exposed along inside diameter of the detector disc, but a gap is normally provided between the shaft and the ring when the shaft is being properly supported by the main bearings, so that the shaft does not contact the ring unless and until the bearings are significantly worn.
During operation, as the main bearings are approaching failure, the main generator shaft will move within the increased internal clearance of the bearings. Eventually, the shaft will rub against the detector disc, and it will quickly rub through a portion of the thin metal ring on the inside of the disc. This will create an open circuit, which is monitored by the control circuit. The control circuit will sense this open circuit as an interruption of the bearing function signal, and will send the appropriate signal to the cockpit. At this time, the generator shaft will be supported by auxiliary bearings, and any further deterioration of the machine operation will be prevented. As the bearings continue to wear, the open circuit will remain, providing a continuous bearing failure signal.
These and other advantages are provided by the present invention of an electrical generator which comprises a housing assembly, and a stator assembly which is fixedly mounted with respect to the housing assembly. A bearing assembly is supported by housing assembly, and the bearing assembly includes main bearings having an internal clearance. A shaft is supported for rotation in the bearing assembly and extends through the housing assembly. A rotor assembly is mounted on the shaft for rotation therewith inside the stator assembly. A bearing failure detector is mounted adjacent to the bearing assembly. The detector senses an enlargement in the internal clearance of the main bearings by a predetermined amount, which enlargement indicates the need for replacement of the bearings.